AE monitoring of corrosion process in cyclic wet–dry test

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Abstract

Recently, a number of deterioration of reinforced concrete (RC) structures due to salt attack have been reported. After corrosion of reinforcing steel-bar (rebar) is nucleated, expansion of corrosion products results in corrosion-induced cracks in RC. Thus, development of non-destructive evaluation (NDE) techniques is important for inspection of corrosion damage. It is reported that acoustic emission (AE) could identify the onset of corrosion in rebar and the nucleation of concrete cracking due to expansion of corrosion products in the corrosion process. In this study, AE techniques are applied to a cyclic wet–dry test of RC beams. It is confirmed that both the onset of corrosion and the nucleation of concrete cracking are clearly observed as two periods of high AE activity. Kinematics of micro-cracks are identified by SiGMA (Simplified Green’s functions for moment tensor analysis) analysis of AE. To compare with findings, cross-sections of rebars are observed by SEM (Scanning Electron Micrograph). From these results, a great promise for AE techniques to monitor the corrosion process in RC structures is clarified.

Introduction

Reinforced concrete (RC) structures are no longer maintenance-free. Especially, damage due to salt attack in RC structures is considered to be one of critical deteriorations in concrete engineering. In the concrete structures, reinforcing steel-bars (rebars) normally do not corrode because of a passive film nucleated on the surface of rebar in concrete of high PH. When chloride concentration at the level of rebar in concrete, however, exceeds the threshold value for corrosion, the passive film is destroyed and corrosion is initiated in rebar. The electro-chemical reaction continues with supplying oxygen and water. Then, due to expansion of corrosion products, corrosion-induced cracks are generated in concrete. Accordingly, development of non-destructive evaluation (NDE) techniques for detection of the corrosion in RC structures at early stage is urgently important. So far, electrochemical techniques of half-cell potential and polarization resistance are widely employed. These techniques estimate corroded conditions of rebars from electrical data.

The relationship between deterioration and life-cycle of a concrete structure is standardized as shown in Fig. 1 [1]. Deterioration process of RC structure due to corrosion is divided into four stages as, dormant, initiation, acceleration and deterioration. Two transition periods are defined at the onset of corrosion and at the nucleation of concrete cracking. The first is the transition period from the dormant stage to the initiation stage. In the former stage, penetration of chloride ions occurs. Following the transition, the electro-chemical reaction is initiated in the latter stage. Corrosion activity continues with supplying oxygen and water. Then, at the second transition, corrosion products are created on the surface of rebar, and corrosion-induced cracks start to be nucleated in concrete. It is normally considered that the electrochemical techniques are available to identify the corrosion process following the initiation stage. In previous research [2], [3], it was reported that AE techniques could provide an earlier warning than the electrochemical techniques.

Elsewhere, according to a phenomenological model of steel in marine environments [4], a typical corrosion loss during the corrosion process is illustrated in Fig. 2. At phase 1, the onset of corrosion occurs. The rate of the corrosion grow is controlled by the rate of transport of oxygen and water. As the corrosion products build up on the corroding surface of rebar, the flow of oxygen is eventually inhibited. Thus, the rate of corrosion loss decreases and is stabilized at phase 2. The corrosion process advances further, and the corrosion loss again increases as phases 3 and 4, due to anaerobic corrosion, where the corrosion penetrates inside the rebar and the expansion of corrosion products occurs. Whereas a monotonous increase is simply assumed in Fig. 1, a two-step corrosion process is modeled.

Based on these findings, continuous AE measurement was conducted to monitor the corrosion process in RC specimens in a laboratory. The SiGMA analysis is applied to a cyclic wet–dry test of RC beams, and kinematics of AE sources during the corrosion process are studied.

Section snippets

Experiments

Reinforced concrete specimens of dimensions 100 mm × 75 mm × 400 mm were made. One deformed rebar of 13 mm diameter was embedded with 20 mm cover-thickness from concrete surface. Configuration of the specimen is illustrated in Fig. 3. The rebar was coated by epoxy except for a target area in the figure. Mixture proportion of concrete is given in Table 1. Because the cover-thickness was set to 20 mm, the maximum size of coarse aggregate was selected as 10 mm. Here, NaCl solution is employed as mixed-water.

Ib-value analysis

In addition to AE activity or occurrence, characteristic of AE signal are conventionally defined by such waveform parameters as rise time, maximum amplitude, counts and duration time. These are shown in Fig. 5. Here, in order to evaluate the size distribution of AE sources, the amplitude distribution of AE hits is taken into account. AE hit is the term to indicate that a given AE channel has detected and processed one AE transient signal.

A relationship between the number of AE hits, N, and the

AE activity

Generating behaviors of cumulative AE hits for all six channels and AE events during the cyclic wet–dry test are shown in Fig. 7. For AE events, which are reasonably located inside the specimen, the number of the hits for 1 h is indicated. Here, generating process of AE hits and AE events observed is classified into two stages, referring to the process of corrosion loss in Fig. 2. AE activity starts gradually to increase at the stage 1 during the first 126 days, and then acceleratedly increases

Conclusion

Continuous AE monitoring was conducted in reinforced concrete specimens under cyclic wet–dry conditions. Results are summarized, as follows:

  • (1)

    From the SEM observation at the stage 1, it is found that a passive film on the surface of the rebar is destroyed. At 28 days, no corrosion is identified from the cross-section, although a little exfoliation is observed at the surface. At 70 days, rust and loss of the oxide film are clearly observed.

  • (2)

    Comparing these SEM photos with AE activity and Ib-value

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